It has been well established that trailer skirts reduce aerodynamic drag of a standard tractor trailer by 5-15% depending on their length. A drawback of a rigid panel as a skirt, is that it is prone to contact with rigid ground structures when driving at slow speed (i.e. curbs, railway tracks, snow banks, camber in the docking ramp . . . ), and can therefore be damaged. A primary object of the present invention is to provide a trailer skirt that is stiff and wind resistant when needed at highway speed, but supple and readily stowable at slow speed. An inflatable has its own inherent problems however in that without a stabilizing structure, it would be misshapen, and vibrate such that it too would have limited longevity. The present invention therefore employs a horizontal rib structure affixed the inflatable bag to solve the shape problem, and inelastic cords anchored to the trailer to solve the stability problem. Andrus U.S. Pat. No. 7,147,270 describes the use of horizontal ribs to shape control an inflatable boattail device, and Andrus U.S. Pat. No. 6,409,252 describes the use of inelastic cords to stabilize a boattail. The boattail bags retain symmetry along the trailer's longitudinal axis by inflating against one another, while the skirt bags of the present invention are free floating and require a specific crossed cord arrangement described herein in order to retain vertical orientation and optimize stability when pressurized. Further to Andrus U.S. Pat. No. 7,147,270 the present disclosure also describes a means of providing a universal joint at structural rib interruptions such that a crumple zone is created at the ends of the bags, while retaining a smooth unbuckled surface at these interruptions when the bags are pressurized. The object here is to allow the bags to yield with little stress when contacted end-on, laterally or from below.